1. Field of the Invention
The present invention relates to a radio communication device, and more particularly to a method for estimating a channel and compensating for error occurring in an estimation procedure in a system where a frequency domain signal processing, such as Orthogonal Frequency Division Multiplexing (OFDM), is performed.
2. Description of the Prior Art
In radio communication environments, received signals have a frequency-selective power spectrum characteristic due to transmission delay caused by a multipath. In order to compensate for such a distortion on channels, various channel estimation schemes have been studied.
Among channels estimation schemes, a Linear Mean Square Error (LMMSE) channel estimation scheme has been widely known as a scheme of minimizing estimation error caused by noise. However, the LMMSE channel estimation scheme has disadvantages in that a large number of calculations are required to obtain the statistical characteristic of a channel, the implementation complexity is high, and a processing delay time increases. That is, it is difficult to employ the LMMSE channel estimation scheme due to the burden of implementing a receiver.
In order to solve such a problem, various schemes modified to have a low complexity are being proposed. A Discrete Fourier Transform (DFT)-based channel estimation scheme, which is one of the modified schemes, has a characteristic of removing a predetermined amount of noise, regardless of the statistical characteristic of a channel. Also, a Correlation Error Cancellation (CEC) scheme has been widely known as a scheme of efficiently removing estimation error, which occurs in the DFT-based channel estimation scheme, without a large increase in complexity. However, such complexity reduction schemes cause serious estimation error when they are employed in an actual radio channel environment.
A representative cause of estimation error is a dense multipath channel environment, and a general radio channel environment, where it cannot be guaranteed that the length of a transmission delay is modeled by integer times of a sample space, may be regarded as a cause of error in channel estimation.
Generally, in an OFDM-based radio communication system for commercial use, a guard band is set within a band available for transmission, in order to minimize interference with adjacent communication channels and to facilitate application of a digital filter. That is, data and a pilot are transmitted through subcarriers within a useful band while no signal is transmitted through subcarriers within the guard band. Such subcarriers are called useful subcarriers and virtual subcarriers, respectively.
FIG. 1 is a conceptual view illustrating a method of setting virtual subcarriers within a guard band. Generally, guard bands are positioned at both ends of a frequency spectrum.
Meanwhile, such a setting of a guard band makes it difficult to implement a channel estimator based on time-domain channel impulse response (CIR) estimation. Generally, the initial channel estimation in an OFDM system is performed through a pilot subcarrier signal, established between a transmitter and a receiver, among useful subcarriers in the frequency domain, wherein various interpolation schemes are used to perform a channel estimation of a data subcarrier location. In addition, various advanced schemes may be additionally employed to minimize estimation error due to noise.
For example, the DFT-based channel estimation scheme is implemented in such a manner as to transform a channel frequency response (CFR) value, which has been obtained from an initial channel estimator, into the form of a channel impulse response (CIR) through Inverse Fast Fourier Transform (IFFT), and then to remove noise. Such a noise removal scheme intends to enhance the channel estimation performance, but may degrade rather than enhance the performance due to a wrong operation of removing even useful channel components in a noise removal procedure in the OFDM system, where a guard band has been set.
In this case, in order to compensate for channel estimation error occurring in the noise removal procedure, a correlation error cancellation (CEC) scheme is employed.
The conventional channel estimation error compensation procedure roughly includes four steps as follows.
In step 1, an initial channel impulse response (CIR) value is stored.
In step 2, a sample position of a CIR having the highest power is estimated. In a repeated execution step, the formerly estimated highest power's position is excluded.
In step 3, correlation error is removed based on the CIR value having the highest power, which has been estimated in step 2. In this step, a non-ideal autocorrelation function is used.
Steps 2 and 3 are repeated until predetermined conditions are satisfied.
However, the conventional error compensation scheme in the channel estimation roughly has two problems.
First, when delay positions of a channel aggregate densely, new error is caused by interference between CIR values of corresponding positions. In this case, when a scheme of removing errors one by one from the channel's position having the highest power, like the conventional method, is used, it is impossible to remove the error caused by the interference. Also, since the employed non-ideal autocorrelation function is set based on integer times sample spaces, the error compensation scheme cannot be successfully performed if the delay positions of the channel do not correspond to integer times of the sample space.
This will now be described in detail.
FIGS. 2A and 2B are views explaining the conventional DFT-based channel estimation method.
FIG. 2A is a view showing an example of power distribution according to delay positions of a channel, which is estimated in a receiving side. In FIG. 2A, the values of Number 10, 20, and 30 represent channel impulse response (CIR) values of a channel, and the signal values of the other positions represent noise components. That is, transmission signals are delayed and received to the positions of Numbers 10, 20, and 30 due to a multi-path channel, respectively, and reception signals have power components corresponding to Number 10, 20, and 30, respectively. Also, random noise is distributed in discrete sample positions along a delay time axis. The conventional DFT-based channel estimation scheme is implemented in such a manner as to set a predetermined threshold value for the purpose of detecting a channel impulse response (CIR) of a channel and removing noise, and to remove all signals having a power component equal to or less than the threshold value in order to enhance the channel estimation performance. In this case, after noise has been removed, only useful channel components remain, as shown in FIG. 2B.
However, FIGS. 2A and 2B are only ideal estimation examples, but in reality, such a result is not obtained due to a guard band and a non-sample-spaced channel delay position. That is, a phenomenon where impulse responses are not in a precisely separated form, but is dispersed to both sides, thereby causing power loss, is caused.
FIGS. 3A and 3B are views explaining the conventional CEC scheme.
FIG. 3A is a view illustrating a non-ideal autocorrelation function dispersed due to existence of a guard band (In a real calculation, only power components are illustrated for convenience of illustration and processing of complex numbers). The function can be estimated through a determined transmission signal structure. FIG. 3B illustrates a power distribution of a real channel which can be observed by a receiving side (wherein noise components are not illustrated for convenience of description). The graph of FIG. 3B is identical to a result of convolution of the non-ideal autocorrelation function with the channel impulse response (CIR) values, shown in FIG. 3A. The channel error compensation by the CEC scheme intends to transform channel components in a dispersed form into a form as shown in FIG. 2B. That is, based on the CIR value (i.e. Number 10) having the highest power, channel components dispersed at both sides are removed in the form of the non-ideal autocorrelation function. Also, with respect to the CIR value (i.e. Number 20) having the second highest power and the CIR value (i.e. Number 30) having the third highest power, the same procedure is successively performed.
However, when delay positions of a channel are adjacent to each other, the CEC scheme causes a wrong operation due to an initial mutual interference. FIGS. 4A to 4C are views showing a first example for explaining a wrong operation in the convention channel estimation error compensation scheme (i.e. the CEC scheme). FIG. 4A illustrates an ideal power distribution of a channel with adjacent delay positions, and FIG. 4B illustrates a power distribution of a channel, which can be observed by a receiving side. FIG. 4B shows a phenomenon where the respective mutually adjacent CIR values (i.e. Numbers 10 and 20) are dispersed to both sides, and cause mutual interference, and such a phenomenon causes initial error at each CIR position. Accordingly, when the CEC scheme is performed based on a CIR value including initial error, channel components existing at both sides are not sufficiently removed. FIG. 4C shows a result of a wrong operation caused by the CEC. Consequently, the channel components remaining around Numbers 10 and 20 cause new channel error. Meanwhile, in the case of the CIR value (i.e. Number 30) which is relatively far away, since initial error caused by the mutual interference is very small, there is few channel component remaining in adjacent positions.
FIGS. 5A to 5C are views showing a second example for explaining a wrong operation in the convention channel estimation error compensation scheme (i.e. the CEC scheme). FIG. 5A shows an ideal power distribution of a channel which does not have a delay position corresponding to integer times sample spaces CIR values (Numbers 20 and 30) of non-sample positions illustrated in FIG. 5A are actually divided into Numbers 21 and 22 and Numbers 31 and 32, respectively, and appear, as shown in FIG. 5B. Also, adjacent channel components are dispersed based on CIR values of Numbers 10, 20, and 30. In this case, CIR values having the highest powers, which can be observed by a receiving side, correspond to Number 10, Number 21, Number 22, Number 31, and Number 32 in regular sequence, and, when the CEC scheme is performed based on the CIR values, new channel error occurs as shown in FIG. 5C.
Therefore, when the conventional channel estimation error compensation scheme is applied in a general wireless channel environment, where it cannot be guaranteed that the length of a transmission delay is modeled by integer times of a sample space, the reception performance is degraded.